Multiple input multiple output (MIMO) systems are wireless communications systems that are specified in resolution 802.11 from the Institute of Electrical and Electronics Engineers (IEEE). A MIMO system that receives a signal Y may compute a channel estimate matrix, H, based on the received signal. The signal may comprise information generated from a plurality of information sources. Each such information source may be referred to as a spatial stream. A transmitting MIMO system may utilize a plurality of transmitting antennas when transmitting the signal Y. A receiving MIMO system may utilize a plurality of receiving antennas when receiving the signal Y. The channel estimate matrix for a downlink RF channel, Hdown, may describe a characteristic of the wireless transmission medium in the transmission path from a transmitter, to a receiver. The channel estimate for an uplink RF channel, Hup, may describe a characteristic of the wireless transmission medium in the transmission path from the receiver to the transmitter.
According to the principle of reciprocity, a characteristic of the wireless transmission medium in the transmission path from the transmitter to the receiver may be assumed to be identical to a corresponding characteristic of the wireless transmission medium in the transmission path from the receiver to the transmitter. However, the channel estimate matrix Hdown may not be equal to a corresponding channel estimate matrix for an uplink RF channel Hup. For example, a noise level, for example an ambient noise level, in the vicinity of the transmitter may differ from a noise level in the vicinity of the receiver. Similarly, an interference level, for example electro-magnetic interference due to other electro-magnetic devices, in the vicinity of the transmitter may differ from an interference level in the vicinity of the receiver. At a transmitter, or receiver, there may also be electrical cross-coupling, for example leakage currents, between circuitry associated with a receiving antenna, or a transmitting antenna, and circuitry associated with another receiving antenna, or another transmitting antenna.
The principle of reciprocity, wherein it may be assumed that Hup=Hdown, may also be based on the assumption that specific antennas at a transmitter or receiver are assigned for use as transmitting antennas, and/or assigned for use as receiving antennas. At the transmitter, a number of receiving antennas, NRX, utilized at the receiver may be assumed. At the receiver, a number of transmitting antennas, NTX, utilized at the transmitter may be assumed. If the assignments of at least a portion of the antennas at the transmitter are changed, the corresponding channel estimate matrix H′up may not be equal Hdown. Similarly, if the assignments of at least a portion of the antennas at the receiver are changed, the corresponding channel estimate matrix H′down may not be equal Hup. Consequently, after reassignment of antennas at the transmitter and/or receiver, the principle of reciprocity may not be utilized to characterize communications between the transmitter and the receiver when Hup does not equal H′down, when H′up does not equal Hdown, or when H′up does not equal H′down.
The principle of reciprocity may enable a receiving wireless local area network (WLAN) device A to receive a signal Y from a transmitting WLAN device B, and to estimate a channel estimate matrix Hdown for the transmission path from the transmitting WLAN device B to the receiving WLAN device A. Based on the channel estimate matrix Hdown, the WLAN device A may transmit a subsequent signal Y, via an uplink RF channel, to the WLAN device B based on the assumption that the channel estimate matrix Hup for the transmission path from the transmitting WLAN device A to the receiving WLAN device B may be characterized by the relationship Hup=Hdown. When the WLAN devices A and B are MIMO systems, corresponding beamforming matrices may be configured and utilized for transmitting and/or receiving signals at each WLAN device.
Beamforming is a method for signal processing that may allow a transmitting MIMO system to combine a plurality of spatial streams in a transmitted signal Y. Beamforming is also a method for signal processing that may allow a receiving MMO system to separate individual spatial streams in a received signal Y.
As a result of a failure of an assumed condition for the principle of reciprocity, a beamforming matrix at the transmitting WLAN device, and/or a beamforming matrix at the receiving WLAN device, may be configured incorrectly. In a transmitted signal Y, from the perspective of a signal associated with an ith spatial stream, a signal associated with a jth spatial stream may represent interference or noise. Incorrect configuration of one or more beamforming matrices may reduce the ability of the receiving WLAN device to cancel interference between an ith spatial stream and a jth spatial stream. Consequently, the received signal Y may be characterized by reduced signal to noise ratios (SNR). There may also be an elevated packet error rate (PER) when the receiving WLAN device decodes information contained in the received signal Y. This may, in turn, result in a reduced information transfer rate, as measured in bits/second, for communications between the transmitting WLAN device and the receiving WLAN device.
In some MIMO systems, a transmitting WLAN device may transmit a plurality of spatial streams based on channel state information at the transmitter (CSIT). The CSIT may be based on feedback information sent from the receiving WLAN device B to the transmitting WLAN device A. Based on the CSIT, the transmitting WLAN device A may compute estimated values for the channel estimate matrix Hdown.
In a typical MIMO system, the transmitting WLAN device A may transmit a plurality of information bits simultaneously via the plurality of transmitting antennas. The transmitting WLAN device A may allocate a portion of the plurality of information bits for transmission via at least a portion of the plurality of transmitting antennas. Each allocated portion of information bits may be error correction coded to enable the receiving WLAN device B to correctly detect the binary values associated with each of the transmitted information bits in a received signal. A received bit for which the receiving WLAN device detects an incorrect binary value may be referred to as a bit error. The rate, as measured over a time duration at which bit errors occur, may be referred to as a bit error rate (BER).
In some MIMO systems, the spatial streams transmitted by the transmitting WLAN device A may be characterized by the bit error rate. An error correction coding method may be characterized by the number of error correction bits that are transmitted with a given number of information bits. The ratio of information bits to the total number of error correction and information bits may be referred to as a coding rate. Spatial streams that are transmitted by a transmitting WLAN device A utilizing larger values for the coding rate, may be associated with higher BERs at the receiving WLAN device B in comparison to spatial streams that are transmitted utilizing smaller values for the coding rate. A larger value for the coding rate may be referred to as a weak coding rate in comparison to a smaller value of the coding rate. The error correction coding may enable the receiving WLAN device B to reduce the BER.
In some MIMO systems that utilize CSIT, the signal strength and/or signal to noise ratio (SNR) may vary among the transmitted spatial streams. In such MIMO systems that utilize higher, or weaker, coding rates, the BER performance may not be acceptable.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.